This invention relates to a mobile communication system and, in particular, to an interference canceller type receiving apparatus for use in a direct sequence/code division multiple access (DS/CDMA) cellular type mobile radio communication system.
As is well known in the art, various multiple access types have been adapted in a mobile radio communication system. One of the multiple access type is a CDMA cellular type. The CDMA cellular type mobile radio communication system assigns to each channel with a particular code, transmits to the same repeater a modulated wave to which a carrier having the same carrier frequency is spectrum-spread with the code, establishes code synchronization in each receiving side, and identifies a desired channel. The CDMA cellular type mobile radio communication system may be called a SSMA (spread spectrum multiple access) cellular type mobile radio communication system.
The CDMA type mobile communication system comprises a plurality of mobile stations and a plurality of radio base stations each of which serves as the repeater. Each radio base station is called a base transceiver station in the art. In addition, each mobile station is referred to as a terminal. As described above, inasmuch as the plurality of mobile station carry out communication using the carrier with the same carrier frequency, it is necessary for the CDMA type mobile radio communication system to be uniform reception energy of an upward communication channel from each mobile station communicating with the radio base station without a position of the mobile station.
In order to be uniform the reception energy in the radio base station, the CDMA type mobile radio communication system carries out transmission power control for the upward communication channel as described in TIA (Telecommunication Industry Association)/EIA (Electronic Industries Association)/IS-95.
The CDMA cellular type mobile radio communication systems are classified roughly into a direct sequence (DS) type and a frequency hopping (FH) type. As is indicated by its name, the direct sequence (DS) type is a type to realize spectrum spreading by directly multiplying a signal to be spectrum spread by a signal having an extremely broader band than that of the signal to be spectrum spread. On the other hand, the frequency hopping type (FH) is a type to realize spectrum spreading by hopping from a frequency to another frequency without fixing a carrier frequency to a particular frequency.
The DS/CDMA cellular type mobile radio communication system comprises a plurality of radio base stations which simultaneously use a carrier having the same carrier frequency. In addition, the DS/CDMA cellular type mobile radio communication system further comprises at least one mobile station which is assigned with its peculiar code (spread code). On transmission, the mobile station widely spreads its own signal by the peculiar spread code to transmit it to a transmission path. On reception, the mobile station receives from one or more radio base stations, as a reception signal, a plurality of path propagation signals which are propagated via different propagation paths. This is because the reception signal is affected by multi-path fading in a transmission path under environment of the mobile communication system.
In order to carry out reception operation at good quality, RAKE reception for separating and combining the different propagation paths is adopted in a DS/CDMA type receiving apparatus. On the RAKE reception, it is necessary for the DS/CDMA type receiving apparatus to follow temporal variations in the different propagation paths. In a conventional DS/CDMA type receiving apparatus, delay-lock loop (DLL) circuits are used as a path follow-up method in the manner which will later be described in conjunction with FIG. 1.
Such a delay-lock loop is described, for example, by Mitsuo Yokoyama in a book published by Kagaku Gijutsu Shuppan Sha, 1988, pages 290 to 311, under the title of "Spread Spectrum Communication Systems." In addition, the DS/CDMA type receiving apparatus using the delay-lock loop circuits is described, for example, in Japanese Unexamined Patent Publications of Tokkai No. Sho 57-65,935 or JP-A 57-65,935, of Tokkai No. sho 63-13,440 or JP-A 63-13,440, of Tokkai No. Hei 6-29,948 or JP-A 6-29,948, or the like.
On the other hand, a multi-access interference becomes an issue in the DS/CDMA system. This counter-measure is, for example, proposed in Japanese Unexamined Patent Publication of Tokkai No. Hei 7-30,519 or JP-A 7-30,519 which discloses a CDMA receiver with less reception characteristic deterioration against increase in the number of simultaneous operation users even in the environment of high speed fading or multi-path in the DS/CDMA system. According to JP-A 7-30,519, a signal subjected to coding multiplex is received by an antenna and band-limited by a reception filter. A signal in the designated timing is inputted to an interference elimination equalizer. Each interference equalizer regards a multi-path component of its own station in other timing equivalently as an other station signal and eliminates the signal together with the other station signal to detect only a multi-path component in the designated timing. Each multi-path component is multiplied with a synthesis weight coefficient and the result is synthesized at a synthesizer and a decision signal is obtained by a decision device.
In addition, JP-A 7-30,519 proposes a method of adaptively controlling tap coefficients in inverse spread filters. This method is called an adaptive interference canceller method which is abbreviated to an AIC method. In the AIC method, the delay lock loop circuits fall into disuse. This is because the AIC method automatically follows slow temporal variations in the different propagation paths by adaptively changing the tap coefficients in the inverse spread filters.
However, the CDMA receiver according to JP-A 7-30,519 is disadvantageous as described hereunder. Firstly, some of a plurality of rake fingers follow the same propagation path during use. Secondly, a departure of following occurs if propagation environment rapidly changes or if the environment of the multi-path dynamically changes. Thirdly, it is difficult to catch a new effective propagation path if it happens under the condition that the propagation environment rapidly changes.
Various other CDMA receivers related to the present invention are already known. By way of example, Japanese Unexamined Patent Publication of Tokkai No. Hei 6-77,928 or JP-A 6-77,928 discloses a spread spectrum communication synchronizing system which is capable of shorten the time up synchronization by detecting a peak with N pieces of matching filters and selecting the maximum of added values. According to JP-A 6-77,928, an inverse spread arithmetic part delays an input signal with a clock at the velocity of N multiple of a reciprocal of a spread rate while using a delay line, where N represents a natural number which is not less than two. A weighting coefficient for inverse spread is multiplied for every N pieces of taps of the delay line. Outputs of the same order in the N pieces of taps are synthesized. Thus, the N pieces of matching filters for performing the arithmetic of inverse spread are formed. On the other hand, N pieces of added value arithmetic parts are provided. Outputs of the respective matching filter are delayed by another delay line. Peak points at the outputs of respective taps in the other delay line are added and outputted. Further, the maximum value of the added value arithmetic part is selected by a maximum value selection part. This maximum value is supplied to the delay lines while controlling clock timing at a clock timing control part so as to increase the value.
Japanese Unexamined Patent Publication of Tokkai No. Hei 7-30,514 or JP-A 7-30,514 discloses a spread spectrum receiver which is capable of eliminating a multi-path interference signal in a received spread spectrum signal for a base band. According to JP-A 7-30,514, a matching filter applies inverse spread spectrum processing to a reception signal subjected to spread spectrum processing by using a code. A transmission line estimate means replies a pilot signal included in the reception signal to estimate a transmission characteristic of a multi-path transmission line to produce a tap coefficient. A transversal filter uses a tap coefficient as a tap weight to produce a maximum ratio synthesis signal of a pulse train. A multi-path interference recovery means responds to the tap coefficient, a demodulation signal and an inverse spread code to recover a multi-path interference signal. A subtracter means subtracts an interference signal from a delayed synthesis signal resulting from the synthesis signal delayed by a delay means and a discrimination means discriminates the subtraction signal to provide the output of a demodulation signal.
Japanese Unexamined Patent Publication of Tokkai No. Hei 7-273,713 or JP-A 7-273,713 discloses a reception equipment, a base station reception system, and a mobile station reception system which are capable of providing demodulated data of a lower error rate by suppressing the influence of interference signal from another station and suppressing the influence of multi-path in the reception system to which CDMA is applied. According to JP-A 7-273,713, a synchronizing signal is detected in a synchronizing signal detection part, and amplitude information and phase information of a main wave and delay wave are generated from this reproduced synchronizing signal. A synchronizing signal eliminating part uses the reproduced synchronizing signal to eliminate the synchronizing signal from the signal from a radio demodulation part and gives an obtained reception signal to another station interference eliminating part. Other station interference eliminating parts use the amplitude information and the phase information to estimate the signals of first through N-th stations while eliminating the intra-station interference from the reception signal. Estimated signals are subjected to correlation detection in correlation detection parts to obtain demodulated signals of the first through the N-th stations.